Nothing there, not even the "__LOCAL" directory. This path does exist, but the file is empty:
D:\Steam\steamapps\common\Kerbal Space Program\GameData\__LOCAL\net.lisias.ksp\UbioWeldingLtd\Aero\weldedpart\weldedpart.cfg
If I copy D:\Steam\steamapps\common\Kerbal Space Program\GameData\__LOCAL to D:\__LOCAL I get a slightly different path in the error message:
D:\Steam\steamapps\common\Kerbal Space Program\GameData\Aero\weldedpart\weldedpart.cfg
If I then copy GameData/__LOCAL/net.lisias.ksp/UbioWeldingLtd/Aero to GameData/Aero, then... the save works!? I restart the game and I can use the welded part.
Then I try to weld the Fuel tanks on the Albatross, and get another error message with this path:
D:\Steam\steamapps\common\Kerbal Space Program\GameData\FuelTank\test\test.cfg
Again, I copy from GameData/__LOCAL/net.lisias.ksp/UbioWeldingLtd/FuelTank to GameData/FuelTank, restart and attempt to weld the fuel tanks again. It works! It's an awkward process, but it works.